Zeolites generally are aluminosilicate oxide structures that have well-defined pore structures due to a high degree of crystallinity. Crystalline aluminosilicate zeolites can comprise both natural and synthetic aluminosilicates. Crystalline aluminosilicate zeolites include those having aluminosilicate cage structures in which alumina and silica tetrahedra are intimately connected with each other in an open three dimensional crystalline network. The tetrahedra are cross linked by the sharing of oxygen atoms, with spaces between the tetrahedra occupied by water molecules prior to partial or total dehydration of the zeolite. Dehydration results in crystals interlaced with channels having molecular dimensions. In a hydrated form, the crystalline aluminosilicate zeolites are generally represented by the formula, M2/nO:Al2O3:wSiO2:yH2O, where “M” is a cation that balances the electrovalence of the tetrahedra and is generally referred to as an exchangeable cationic site, “n” represents the valence of the cation, “w” represents the moles of SiO2, and “y” represents the moles of water. The exact structure type aluminosilicate zeolite is generally identified by the particular silica, alumina molar ratio (SiO2/Al2O3) and the pore dimensions of the cage structures. Cations (M) occupying exchangeable cationic sites in the zeolite may be replaced with other cations by ion exchange methods well known to those having ordinary skill in the field of crystalline aluminosilicates.
Zeolite crystalline particles may be formed from zeolite fine powder mixed with a binder. The binder may be an amorphous inorganic material, such as silica, alumina or certain clays and mixtures thereof “Formed zeolites” may be extrudates, tablets, oil drops, microspheres, spheres, such as beads, or the like. The zeolites may be formed by oil-dropping, spray-drying, extrusion, or other “forming” techniques.
Recently, nanosized zeolites or nano zeolites (less than about 300 nm) have attracted considerable attention because of their potential advantages in catalysis due to their high external surface area, reduced diffusion path lengths and exposed active sites. The reduction of particle size from the micrometer to the nanometer scale leads to substantial changes in the properties of materials, which have an impact on the performance of zeolites in traditional application areas such as catalysis and separation. The ratio of external to internal number of atoms increases rapidly as the particle sizes decrease. Additionally, the nano zeolite crystals have reduced diffusion path lengths relative to conventional micrometer-sized zeolites.
Nano zeolites are typically formed by crystallizing (e.g. reacting) a gel that comprises a source of silica and alumina, sodium hydroxide, a structure directing agent also known as a template, and water at temperatures of about 100° C., for example. In general, the nano zeolite formation reaction typically requires a substantial excess of silica, alumina and sodium hydroxide that remain in the crystallization aqueous suspension, also known as “mother liquor”, in various forms (e.g. residuals comprising unreacted components, salts, etc.) together with the crystallized nano zeolites. Because of the nano-size of the nano zeolites, it is often difficult, expensive and time-consuming to remove the nano zeolites from the mother liquor including separating them from the residuals. Conventional filtration methods for separation are impractical due to the nano size of these particles. Rather, costly high speed commercial centrifuges operating at from about 50,000 to about 125,000 revolutions per minute (RPM), for example, are used for the separation process. A high speed centrifugal force is applied to the mother liquor which separates into a supernatant phase and a solid phase. The solid phase, which contains the nano zeolites and the residuals, is collected and incorporated into a liquid wash medium and the combination is again subjected to high speed centrifugal force to remove a portion of the residuals in the liquid wash. This process is repeated many times, unfortunately often over the course of several days or weeks, until the residuals have been substantially removed leaving primarily the nano zeolites. Thus, preparation and recovery of nano zeolites is very time consuming and expensive.
Accordingly, it is desirable to provide a more efficient process for recovery of the nano zeolites from the mother liquor which is less time consuming and does not require the use of costly high-speed commercial centrifuges. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.